Trunk diameter influences attack by Polygraphus proximus and subsequent mortality of Abies veitchii

Abstract

Polygraphus proximus Blandford (Coleoptera: Scolytidae), a non-aggressive phloephagous bark beetle that feeds on Far Eastern fir species, is the main species responsible for the rapid and widespread destruction of natural Abies sibirica forests, and has caused the mass mortality of planted Abies veitchii trees in Japan. It is unclear how long the attacks continue or whether the beetles progress to larger-diameter trees as their attacks progress. Thus, we investigated P. proximus attacks on A. veitchii and the association between tree mortality and trunk diameter over 6 years in Japan. Trees were severely attacked, and the proportion of infested trees significantly increased over 6 years. Small-diameter trees were attacked significantly more severely than large-diameter trees, and the beetles attacked larger-diameter trees in year 6. Despite this increase, larger-diameter trees were better able to survive attack. These results suggest that the beetles initially attack smaller-diameter trees in the early phase of mass mortality and then gradually attack larger-diameter trees as the infestation advances, but that the larger-diameter trees better survive the infestation. Enhanced colonization of smaller-diameter trees by P. proximus may act as a trigger for mass mortality and result in higher tree mortality; however, larger-diameter trees might regenerate A. veitchii forests after mass mortality.